This invention relates to capacitors formed of tantalum or the like, and especially to tantalum capacitors of the type wherein the anode is tantalum metal, an insulating compound such as tantalum oxide acts as the dielectric and, in a common arrangement, a conductive package in which the tantalum oxide is carried acts as the cathode. Such capacitors are generally described in a number of patents, including:
U.S. Pat. No. 3,585,027 PA0 U.S. Pat. No. 3,553,805 PA0 U.S. Pat. No. 3,600,796 PA0 U.S. Pat. No. 3,585,072 PA0 U.S. Pat. No. 3,653,119
In general, the manufacturing processes for such tantalum capacitors are rather complex. For example, in a typical process, an anode is made by forming, around a tantalum wire, a plug of pressed tantalum powder; sintering the plug into a porous metal part; converting the surface of the tantalum to tantalum oxide; soaking the plug with Mn(NO.sub.3).sub.2 solution; heating and drying thereof; again treating the surface of the plug to assure any metal is oxidized. The oxidized layer of tantalum may be considered the dielectric member of the capacitor. The tantalum is the anode and maganese dioxide is believed to form part of the cathode.
Thereupon, the tantalum oxide-coated plug described above is soaked in, i.e. impregnated with, a liquid dispersion of ultra-small particles of conductive carbon. After drying, the carbon-soaking step is repeated. Thereupon the part is dipped into a silver paint such as that sold by DuPont under the trade designation 8079 silver paint. After dipping, the part is usually dried and dipped once again. The silver paint serves as a solderable conductor.
The silver-coated device described above is slid into a can. Usually the can contains a disc of solder metal therein, and heat is used to melt the solder, thereby assuring good electrical connection between the silver paint forming part of the cathode on the tantalum capacitor and the can itself. An anode lead runs from the tantalum wire which was only partly embedded in the porous plug; a cathode lead runs from the metal can which has now been incorporated into the "cathode side" of the capacitor.
Alternately, the silver-coated device is wetted with solder; a cathode lead being soldered to the device during the soldering process; and then the device is encapsulated within an organic resin, usually an epoxy. The cathode and anode leads are, of course, left exposed.
Satisfactory capacitors were made by such a process as described above, but it was most desirable to (1) improve their electronic characteristics and/or (2) provide a relatively inexpensive way for manufacturing such devices.
Consequently, it is an objective of the Applicant's invention to achieve both of these advantages, and preferably, to do so simultaneously by providing both an improved capacitor product and an improved process for making the product.